Chain saws and brush cutters are known as portable work apparatuses for logging or trimming trees, or mowing grass. Some of compact work apparatuses of this type use electric motors as their drive sources. Most of them, however, are equipped with internal combustion engines (that are typically single-cylinder two-stroke engines or single-cylinder four-stroke engines), and are generally configured to transmit the engine power to the cutting element via a centrifugal clutch (Patent Document 1).
In addition, work apparatuses of this type generally use a carburetor (Patent Documents 2 to 4). Patent Documents 2 to 4 disclose technologies related to starting an engine in a cold state or in a hot state. More specifically, Patent Document 2 discloses a mechanical linkage between a throttle control trigger to be operated by an operator for control the engine output combined with a selector (substantial choke knob) and a throttle valve combined with a manual choke valve. Patent Document 3 discloses a mechanical linkage between a throttle valve and a manual choke valve. Similarly, Patent Document 4 discloses a mechanical linkage between a throttle valve and a manual choke valve.
A procedure for starting an engine in a cold state is explained below.
(1) A choke knob is operated to set the choke valve at a full-shut position. Responsively, a throttle valve is positioned and held at a “first idling position”.
(2) In case the engine has a recoil starter as its trigger device, a starter grip of the recoil starter is pulled several times to feed a cylinder with fuel-rich air-fuel mixture, and the pulling operation is repeated until explosion occurs in the cylinder. In general, the air-fuel mixture fed by this operation is too rich to continue the explosion. Therefore, the engine does not continue to rotate, and stops after several cycles of explosion.
(3) The choke knob is next operated to return the choke valve at its full-open position. The throttle valve is maintained at the “first idling position”. Under the condition, the starter grip of the recoil starter is pulled again. Thus, the engine gets in continuous rotation.
(4) When a throttle control trigger is operated, linkage between the throttle valve and the choke valve is cut off, and the throttle valve takes a position of an opening degree in accordance with the operation of the throttle control trigger. In other words, the throttle control trigger and the throttle valve are mechanically linked, which results in producing an engine output corresponding to the operation of the throttle control trigger. Then, the throttle control trigger is released, and the throttle valve is accordingly brought to and held at a “normal idling position” that is a nearly full-shut position. Therefore, operation of the throttle control trigger after starting the engine can be regarded as an operation for cutting the linkage between the throttle valve and the choke valve.